When a nss.dropDatabase() command gets executed on a sharded collection, every shard participating in the DDL is currently adding an internal {op: drop}  event to its oplog for each collection that gets locally dropped.

This may generate some ambiguity from the perspective of a mongos serving a droppedDb.coll.watch() request (which is collecting oplog events from multiple shards) to establish the exact point in time at which the change stream may be considered as invalidated.

To solve this, each non-primary shard should hide the "local collection drop" event by marking the related oplog entry with the fromMigrate option.